It is known to control the movement speed of one or more train vehicles moving along a track including a plurality of signal blocks through the use of speed code signals in accordance with desired fail-safe control system operation. Prior art discloses of similar railway track signaling systems can be found in U.S. Pat. No. 3,562,712 of G. M. Thorne-Booth et al, U.S. Pat. No. 3,551,889 of C. S. Miller and U.S. Pat. Nos. 3,532,877 and reissued 27,472 of G. M. Thorne-Booth. In addition, an article entitled "Design Techniques For Automatic Train Control" by R. C. Hoyler in the Westinghouse Engineer for July, 1972 at pages 98 to 104 and an article entitled "Automatic Train Control Concepts Are Implemented By Modern Equipment" by R. C. Hoyler in the Westinghouse Engineer for September, 1972 at pages 145 to 151 describes train control equipment design for safe operation.
For the purpose of starting and stopping the movement of a train, it is desired to know the value of the adhesion coefficient of friction between the train wheels and the track rail upon which the train is moving. The operational speed permitted for a second train following an earlier first train is determined in part by the location of the first train on the track ahead of the second train as well as the adhesion condition of the track ahead of the second train. If it is desired to stop the second train a predetermined safe distance behind the first train with no concern about a collision between the two trains, it is necessary to determine a safe distance to begin stopping the second train behind the first train. This distance is a function of the adhesion level and the resulting deceleration rate that can be reasonably achieved by braking the train in relation to the train velocity. In general this stopping distance D can be determined by the relationship: EQU D=V.sup.2 /2R (1)
where V is the train velocity and R is the known deceleration rate. If rain, ice or some other film material is present on the track, this will change the deceleration rate R.
It is known to provide wheel slip or slide detection and control apparatus to remove a command for propulsion or braking of the train until the abnormal adhesion situation is corrected. The tractive or braking effort being applied to the wheel axle must be corrected to permit the wheels to regain the speed equivalent of the train speed. A speed signal is developed for each axle by suitable speed sensors with a D.C. output voltage proportional to frequency of the output of the speed sensor being developed and a derivative of this voltage being used to trigger a slip slide control system, as described in a publication entitled "Propulsion Control For Passenger Trains Provides High Speed Service" by J. E. Moxie et al in the Westinghouse Engineer for September, 1970, at pages 143 to 149. When the derivative voltage exceeds a value equivalent to wheel acceleration or deceleration of 8 miles per hour per second, an output from the slip slide control system picks up a relay to initiate action that reduces the tractive or braking effort until the slip or slide is eliminated. Once the slipping or sliding condition has been eliminated the wheels should return to a speed equivalent to train speed. An assumption can be made that the return to speed will be in the order of the rate of 8 miles per hour per second or greater, therefore a speed derivative with a sign opposite the sign of the derivative signal that initiated the reduction in tractive or braking effort can be used to reset the control system, eliminate the system output, drop out the control relay and reestablish the desired braking or tractive effort. It is recognized that the assumption that an equivalent wheel deceleration always follows a wheel slip acceleration and vice versa may not be valid and for this reason a time out circuit is provided with timing initiated when a system output signal occurs. If the opposite sign derivative does not occur before the end of three seconds, the control system is reset and the relay dropped out by the timing circuit which relay drop out then permits the desired tractive or braking effort to be reestablished.